While forming heat seals between films made from low-temperature thermoplastics, such as PET, LDPE, MDPE, and HDPE, is well known, current technology does not permit the formation of heat seals between films made from high-temperature materials or between high- or low-temperature films and non-planar structural elements, such as molded thermoplastic valves.
One class of high-temperature materials that are not currently capable of being heat-sealed is high-temperature thermoplastics. Foremost among high-temperature thermoplastics is a subclass generally referred to as "liquid crystal polyesters" or "liquid crystal polymers" (hereinafter "LCP"), that, when formed into films, exhibit superior heat tolerance and impermeability characteristics. There is widespread interest in LCP films because, unlike films made from other high-performance polymers, such as polyetheretherketone (PEEK), various polysulfones (PSFs), and polyetherimide (PEI), which are based on exotic and inherently expensive chemistries, LCP films are based on polyester chemistry, which is well understood in resin or extruded form. Thus, although LCP films are currently expensive due to their limited usefulness, their cost is expected to decrease with increasing demand.
LCP films are suitable for the most demanding applications. They exhibit low moisture absorption, inherent flame retardance, high thermal stability (e.g., Amoco Performance Products's XYDAR.RTM. SRT-300, an unmodified commercial LCP, has a 1.82 MPa heat distortion temperature of 355.degree. C., higher than any known conventional engineering thermoplastic even with the benefit of fiber reinforcement), high radiation resistance, and ceramic-like resistance to chemical attack and reagent permeation.
LCP films offer unsurpassed barrier performance among polymeric materials, especially in critical applications, such as food and pharmaceutical packaging, fiber-optic jackets, and fuel tanks. The barrier properties exhibited by LCPs are orders of magnitude greater than those of conventional barrier resins, such as EVOH, PET, metallized PET (MYLAR.RTM.), and PVDC, the latter being the standard-bearer of general purpose barrier polymers. E.g., a film prepared from Hoechst Celanese VECTRA.RTM. resin is ten times more resistant to oxygen and water permeation than PVDC and 100 times more resistant to oxygen permeation and almost twice as resistant to water permeation than PCTFE, which itself is among the best of all conventional barrier polymers.
Similarly, and unlike traditional structural packaging materials (i.e., co-extruded films), such as LDPE, MDPE, HDPE, PP, and PET, LCPs offer thermal stability sufficient for hot-filling and retorting (up to about 200.degree. C.) in food packaging and are sterilization-compatible in addition to their superior barrier properties. LCP films are likewise suitable for medical packaging of appliances and devices, biofluids, and pharmaceuticals due to their ultra-low permeability, biocompatibility, and sterilization compatibility. Finally, LCP films are also superior to polyimide films, such as Du Pont's KAPTON.RTM., in electronic substrate applications, such as in flexible circuits.
However, because of their high melting points-typically about 260.degree. C. (c. 500.degree. F.) to about 425.degree. C. (c. 800.degree. F.)--and complex linear molecular structure, LCP films exhibit poor adhesion/cohesion properties. Indeed, all thermoplastic films that have a melting point greater than about 260.degree. C. (c. 500.degree. F.), and/or chemical properties which make them difficult to reconsolidate using temperature and pressure, and a total thickness greater than 0.080", are difficult, if not impossible to join. The only exception among these materials is PVC, which can be RF or UV welded due to its unique electrical properties. As a result, there is currently no commercially available processes or machines that can accomplish a viable weld or seal of films made from most high-temperature thermoplastic materials due to the extreme temperatures required (above about 500.degree. F.) and, accordingly, there are no products manufactured from high-temperature thermoplastic films. Various forms of cold adhesion, including ultrasonic bonding, are also ineffective for sealing high-temperature thermoplastic films such as liquid crystal polymer.
Several forms of heat-sealing apparatus for sealing products comprising low-temperature thermoplastic films, with thermal management, are disclosed in U.S. Pat. Nos. 4,075,818 issued 28 Feb. 1978 to Wright et al. and, 4,359,361 issued 16 Nov. 1982 to Wright.
The devices described in both the '818 and '361 patents comprise opposed sealing jaw assemblies, one of which is a backup jaw assembly and the other of which is a heat applying jaw assembly, the latter including an elongated sealing and cutting element which is workable against the former. The devices further comprise means to heat the sealing and cutting element and, in at least one of the jaw assemblies, one or more cooling veins disposed generally immediately adjacent the working face of the at least one jaw assembly and generally contiguous to and along the sealing and cutting element, at least in the closed jaw position. The one or more cooling veins are in fluid communication with, in various embodiments, a stream of pressurized gas, refrigerated gas, or dual streams of cooled gas originating from a heat exchanger having a cooling coil that cools one gas stream and an exhaust means in fluid communication with a vortex tube that cools the other gas stream.
These devices have several characteristics which lead to problems. First, the preferred release material covering the jaw assemblies is glass tape or cloth coated with polytetrafluoro-ethylene (PTFE), which is sold under the trademark TEFLON.RTM.. PTFE is a well-known solid lubricant and is used in the '818 and '361 patents to prevent adhesion to the heat sealing element of the thermoplastic film being heat sealed. Although the PTFE coating allows for non-stick heat sealing of common, lower temperature thermoplastics, such as polyethylene and its derivatives; polyvinylic materials, such as polyvinyl alcohol (PVA) and polyvinyl chloride (PVC); and hybrid materials, such as ethylene vinyl acetate (EVA), it cannot be used in the manufacture of heat-sealed high temperature film products, as it is an ineffective release material at about 260.degree. C. (c. 500.degree. F.) and melts at 327.degree. C. (c. 620.degree. F.) and vaporizes well below the higher LCP melting point of high performance LCPs with melt-processing temperatures of about 425.degree. C. (c. 800.degree. F.). A related drawback of these devices is their inability quickly to dissipate the extreme amounts of heat needed to seal high temperature films, such as those made from LCP.
A general limitation in all relevant prior art devices is their inability to manufacture products comprising high temperature film material. A related limitation is the failure of known devices to incorporate valves, probes, leads, tubes, and the like into a heat-sealed product such that an hermetic seal is formed between the film layers and the incorporated element.
Accordingly, it is an object of this invention to overcome the above illustrated inability of extant devices to heat-seal film materials manufactured from thermoplastics having melting points in excess of about 260.degree. C. (c. 500.degree. F.) by providing heat sealing apparatus capable of changing the material to be sealed to its liquid phase in the seal region and reconsolidating the material to form the seal.
It is another object of this invention to provide heat sealing apparatus that does not experience thermal breakdown at the high temperatures required to heat seal LCP film layers having a thickness totaling about 0.080".
It is yet another object of the present invention to provide high-temperature heat sealing apparatus that enables the sealed product to fully release from the sealing element after sealing is complete without damaging the seal.
It is a further object of the present invention to provide apparatus capable of producing a heat seal with and/or without cutting the material to be sealed.
It is yet a further object of the invention to provide heat sealed products comprising high-temperature thermoplastic film material, such as made from LCP resin.
It is a still further object of the present invention to provide heat-sealed bags made from high temperature films and having valves, probes, leads, tubes, and the like incorporated into the heat-sealed bag such that an hermetic seal is formed between the film layers and the incorporated element.